Septic joints can have an insidious onset and are difficult to diagnosis. Diagnosis can be more complicated in the setting of a distant prosthetic joint. Plain films and inflammatory markers are not specific and magnetic resonance imaging (MRI) is not a timely test in the emergency department. Computed tomography (CT) scan is quick and useful to evaluate for bony changes or signs of inflammation, but lacks the details of MRI, and the prosthetic joint may cause significant artifact. Point-of-care ultrasound (POCUS) is often used in the pediatric population to evaluate for an effusion when there is a concern for a septic native hip joint and is finding a role in adult emergency medicine to evaluate for an effusion in painful native adult hip joints. Even so, ultrasound is not currently included in diagnostic algorithms for diagnosing prosthetic hip joint infections (PJIs). POCUS is, however, readily available in the emergency department. We present a case where POCUS aided in identifying a periprosthetic synovitis and changed the course of the patient's management from previous physical therapy to an investigation toward the final diagnosis of a septic prosthetic hip joint.
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